This invention relates to additives for coating compositions and coating compositions containing them.
In general, the surfaces of coating films formed by application of various coating compositions are deficient in that they tend to sustain injuries, stains or haze when kept in contact with any other surfaces. This unfavorable tendency is particularly prominent in case the coating films have a comparatively low hardness. It is known that overlaid plywood coated with a pyroxyline lacquer or an aminoalkyd resin enamel or steel plates coated with an acrylic resin enamel when put one on another or as rolls with individual coated surfaces in contact, tend to have injuries or haze incurred in the coating layers. It is also known that when a pressure-sensitive adhesive tape is pressed onto the coated surface and then peeled off, part of the coating layer transfers to the contacted surfaces of the adhesive tape, resulting in the breakage of the coating surface. This sort of phenomenon is called blocking.
In the prior art, improvements have been proposed in order to eliminate or mitigate the blocking phenomenon for example, by adding to coating compositions a small amount of a dimethylsilicone fluid, organopolysiloxanepolyoxyalkylene copolymer, aminoalkylsiloxane-dimethylsiloxane copolymer, or the like (see, for example, Japanese Patent Publications No. 18985/'72 and 4367/'73). However, the addition of the dimethylsilicone fluids can not sufficiently prevent the blocking phenomenon and may cause the unfavorable phenomenon of cratering to the coated surfaces, although it advantageously serves to increase the leveling effect of coatings and prevent the separation of pigments, which is usually called flooding. The addition of the copolymers, on the other hand, serves to obviate the possibility of the cratering phenomenon, but not to sufficiently prevent the blocking phenomenon. For example, blocking takes place when the pressing and peeling movements of a pressure-sensitive adhesive tape on and off the surface of the coating layer are continuously repeated, or when overlaid plywood is given a hot press treatment in the course of manufacture.